No.
40 - Fall 1997

Lord
of the Rings

Terence
Dickinson
SkyNews

It would be the
ultimate backpacking trip: an exploration of Saturn's rings. The astronauts
squeeze into their space suits, strap on their rocket-powered backpacks, and
load their cameras. Their spaceship brings them within a few kilometers of the
rings. So long as the ship remains in a circular orbit, its speed is identical
to that of the chunks of ice and rock which make up the rings. A collision with
any ring material under these conditions is just a gentle nudge.

Stepping outside,
the backpackers -- using brief rocket bursts -- slowly glide toward the glittering
golden plain. The explorers touch down on a large boulder, and in one smooth
push of the foot, propels themselves onto the next big piece. In between this
slow-motion ballet toe-step, the explorers feel the blizzard of tiny particles
gently bouncing off the front of their space suits.

Floating in the
silvery gravel is intoxicating. The partly hidden Sun glinting off the field
of ring chunks, the gentle gravitational symphony of collective motion that
carries both chunks and explorers safely around the planet, the feeling of being
surrounded yet freely floating -- all disguise the fact that everything is whirling
around Saturn at tens of thousands of kilometers per hour.

You don't have
to travel millions of kilometers to visit Saturn. It comes to you. Of all the
celestial sights available to backyard telescopes, only Saturn and the Moon
are sure to elicit an exclamation of delight from those who have never looked
through a telescope before. And one look is seldom enough. No photograph or
description can duplicate the beauty of the colossal ringed planet floating
against the black velvet of the night sky.

Dark
side of the ringed planet. This is most people's favorite picture of Saturn.
With the Sun off to the right, Saturn casts a shadow on its rings; its
daytime hemisphere looks like a crescent. To see a planet or moon as a
crescent, you need to be looking from the side -- a perspective on Saturn
which Voyaer 1 achieved in 1980, but which is impossible from Earth. Photo
courtesy of NASA JPL.

People have been
watching Saturn for millennia, but Galileo Galilei was the first to point a
telescope at the planet and see its rings. From his discovery, in 1610, until
the 19th century, astronomers debated whether Saturn's rings were a solid disc
or a swarm of objects. In a telescope, the rings look solid, yet Saturn's gravity
should tear a solid structure apart. American astronomer James Edward Keeler
resolved the dilemma in 1895. Using a spectroscope to study sunlight reflected
off different parts of the rings, he found that they do not all move at the
same speed, as they would if they were solid. Instead, the parts closest to
Saturn are moving faster than the parts farther out. Keeler concluded that the
rings must consist of individual objects revolving around Saturn just like tiny
moonlets.

These objects
range from tiny crystals, like those in an ice fog, to flying icebergs. Each
has its own orbit about Saturn, but occasionally jostles its neighbors. The
gentle collisions gradually grind down the larger particles. Meanwhile, the
smallest particles tend to stick to one another and create larger clumps. These
competing actions have established an equilibrium of sizes. For every house-sized
boulder, there are a million baseball-sized chunks and trillions of sand-sized
grains.

In the denser
rings, the baseball-sized particles are separated by a meter or so; the house-sized
ones are kilometers apart. In fact, most of the rings are empty space. If they
could be melted and refrozen as a solid body, they would be a solid disc less
than 2 feet thick.

The rings are
enormous. From one edge to the other, they span a distance equivalent to two-thirds
of the gulf between Earth and the Moon. Yet the ring particles seldom stray
more than a few hundred meters from a perfectly flat plane, making the rings
the height of a 30-story building. If the rings were the size of a football
field, they would be paper-thin.

The reason why
the rings are so flat -- rather than a random haze -- has to do with Saturn
itself. Saturn is the least dense of the gaseous giant planets, yet a day on
Saturn is only 11 hours long. This rapid rotation has bulged the planet at the
equator and compressed it at the poles. As a result, there is more material
at the equator, so the gravity is stronger there. A body orbiting Saturn feels
a greater gravitational pull as it passes over the equator, compared with the
poles. Over time, this difference distorts the orbits of the ring particles,
causing them to collide and settle into a circular orbit above the equator.

Although astronomers
are still trying to determine exactly where the rings came from, they think
a collision, either between two of Saturn's moons or between a moon and a comet,
blasted debris into orbit around the planet. The debris became the rings; it
could not regroup into a moon because, near the planet, gravity rips large objects
to shreds. Some scientists think the rings are less than a billion years old
-- fairly young by astronomical standards -- while others think they date back
to the early days of the solar system.